1. Field of the Invention
The present invention relates to a method and a device for correcting the rotation of a video display.
2. Discussion of the Related Art
Generally, a video image is displayed on the screen of a display terminal by exciting phosphors arranged on the screen by means of one or several electron beams, emitted by electron guns. In the case of a color screen, a cathode-ray tube with three electron beams, each exciting a type of phosphor respectively emitting a red, green, or blue light, is generally used. The electron beams are modulated in intensity by modulation signals representative of the image to be displayed on screen.
Generally, the electron beams are focused at one point of the screen and are deviated together to scan screen lines. The electron beams scan the screen lines from the right to the left of the screen, returning to the left of the screen after the scanning of each line. The screen scanning is performed from the upper horizontal edge to the lower horizontal edge.
The electron beam deviations are obtained by two deflection coils, one horizontal deflection coil which controls the scanning of each screen line, and one vertical deflection coil which controls the deviations of the electron beams in the vertical direction.
The deflection coils may have certain defects of manufacturing and/or assembly on the display terminal. The magnetic fields of deviation of the electron beams induced by the deflection coils may then be such that the screen lines scanned by the electron beams are not perfectly horizontal while remaining parallel to one another. The image is then displayed on screen with a rotation angle equal to the inclination of the scanned lines.
A similar phenomenon occurs when the display terminal is placed in an external magnetic field, for example, the terrestrial magnetic field. The external magnetic field may disturb the electron beam deviation so that the scanned screen lines are slightly inclined with respect to the horizontal direction while remaining parallel to one another. The image is then displayed on screen with a rotation angle equal to the inclination of the scanned lines. The inclination angle of the lines depends on the orientation of the magnetic field and may vary if the display terminal is moved.
A conventional solution to this problem is to place a correction coil between the deflection terminals and the rear surface of the cathode-ray tube. The magnetic field induced by the correction coil is adjusted so that the scanned lines of the screen are horizontal. In other words, the correction field induced by the correction coil rotates the displayed image by a correction rotation angle, the sign and the amplitude of which are defined by the polarity and the intensity of the current flowing through the correction coil. The display terminal may include means for setting the current polarity and intensity to enable correcting the rotation defect once the display terminal has been installed.
The correction coil and the coil supply system amount to a non-negligible cost of the display terminal.